I. Field of the Invention
This invention relates generally to medical electronic equipment for assessing cardiopulmonary performance during exercise and for evaluating pulmonary function during static testing. More particularly, the invention relates to an improved flow measuring system which is operative over a wide dynamic range of respiratory flows as is seen in patients with differing degrees of respiratory impairment.
II. Discussion of the Prior Art
Differential pressure pneumotachographs have been in use for several decades. Typically, these devices consist of a tubular, open-ended frame with a known value flow resistive element inserted in the lumen thereof. The resistive element is generally either one or more screens positioned transverse to the direction of gas flow or a grouping of parallel capillary tubes within the gas flow. Under conditions of gas flow, this creates a pressure drop across the resistive element which can be assessed by connecting pressure taps at sequential points along the tube with a differential pressure transducer. As an example of this type of pneumotachograph is described in the Anderson et al. U.S. Pat. No. 4,463,764, the Rudolph U.S. Pat. No. 3,626,755 and published results of Fleisch (Pfluegers Arch. 209: 713-722, 1925), Lilly (Methods of Medical Research. Chicago, IL; Yearbook, 1950, 2:113-121), Pearce et al. (J. Appl. Physiol: Respirat. Environ. Exercise Physiol. 42: 968-975, 1977) and Osborn (Crit. Care, Vol. 6, No. 5: 349-351, 1978).
Although widely accepted for use, these types of pneumotachographs suffer from several problems as it relates to accurately measuring inspiratory and expiratory flow. To maintain a linear relationship between flow and the pressure drop, the resistive element must maintain laminar flow. Failure to maintain laminar flow in these types of pneumotachographs generates unpredictable linearity. These resistive elements create a back resistance to flow which can distort the measurements, particularly in patients with significant respiratory impairment. Moreover, back resistance to flow can distort the measurements. The frequency response of the pneumotachograph is important since if the change in pressure across the resistive element is out of phase with the actual flow signal, it has important significance when performing phase alignment for gas analyzers and flow signals during gas exchange measurements.
Further, after a short period of use, the screens or capillary tubes become coated with condensation and/or saliva which invariably alters the resistance value. Heating the pneumotachograph to prevent condensation helps somewhat, but complicates the calculations by cooling the resistive element unpredictably as gas flow changes. Because it is a wetted surface, the device must be replaced or decontaminated between patients. The design of the resistive elements creates a relatively high cost item which is at odds with disposability. Decontamination is both time-consuming and inconvenient as the resistive element must be thoroughly dired after cleansing.
Devices which do not employ resistive elements produce changes in pressure as a function of the square of the flow. Pressure measurement over the dynamic range dictated by patients having varying degrees of respiratory impairment.
As recommended by the various organizations such as the American Thoracic Society, American College of Chest Physicians and the National Institute for Occupational Safety and Health, measurements made from patients should be corrected to a standard environmental condition, specifically fully saturated, body temperature and pressure. Traditionally, this has been accomplished by assuming that the respiratory gases cool to ambient room temperature and applying a fixed correction of approximately 8%. It is widely known that gas cools dynamically depending upon the expiratory flow. Gas measured during high flows will more closely approximate body temperature than during low flows. This means that during high flow, the correction will be smaller than during low flow. The magnitude of this error can approach 5%. It is apparent to those skilled in the art, that a dynamic correction based upon the actual measured temperature is preferable.
Another drawback of the Hans Rudolph pneumotachograph mouthpiece is that it includes a significant dead-space leading to inaccuracies due to the patient rebreathing previously expired gas sample. This, too, distorts the readings obtained from any O.sub.2 or CO.sub.2 analyzer which may be coupled to the mouthpiece.
A further drawback of the prior art mouthpiece is that it tends to be a relatively high cost item. Because it embodies wetted surfaces, i.e., the screen(s) and tubular housing, it is treated as a disposable unit to avoid the possibility of the spread of harmful virus from patient-to-patient. High cost and disposability run at odds to each other.
Those skilled in the art will appreciate that when applied to respiratory gas analysis systems used in the evaluation of cardiopulmonary performance, the flow measuring system must be capable of operating over a broad dynamic range so as to be operative with patients with both healthy and sick pulmonary organs and with adults as well as infants and children.